1. Field of the Invention
The invention relates to emergency call establishment in a communication system. Particularly, the invention relates to a method for the obtaining of position information for a user initiating an emergency call in a communication network.
2. Description of the Related Art
The obtaining of calling subscriber or user position during the establishing of an emergency call is invaluable. Ever since the introduction of digital switches it has been technically easy to obtain at least a calling line identity for the use of an emergency call answering center. In mobile communication networks where the calling subscriber identity itself usually tells nothing about the actual location of the calling subscriber there has been defined the possibility for the retrieval of calling user location. The location may be a cell identifier or even a geographic location obtained by the use of a satellite positioning system interface in the terminal or approximated distances to a number of base stations. In some cases the calling user location may be used to route an emergency call to a proper destination emergency call answering center, for example, a Public Safety Answering Point (PSAP).
3G Partnership Project (3GPP) specifications define an IP Multimedia Subsystem (IMS) that provides for multimedia session establishment over various Internet Protocol (IP) networks. Multimedia session comprise, for example, audio and video calls, instant messaging, and audio and video streaming. IMS also supports user presence sharing that enables other users to determine the status of other users. The IMS may be used not only from mobile terminals in the area of a cellular communication system that supports roaming in a uniform manner, but also from Wireless Local Area Networks (WLAN) and fixed IP networks. The fixed IP networks may provide connectivity for user terminals via a Local Area Network (LAN) or a Digital Subscriber Line (DSL) connection. An IMS user may obtain connectivity to the IMS via different IP Connectivity Access Networks (IP-CAN). The IP connectivity access for a user may be based on a gamut of different technologies. IP-CANS comprise, for example, the General Packet Radio Service (GPRS), WLANs and fixed IP networks. The different IP-CANS support very different ways for the positioning of a user. Some IP-CANS may not support any positioning of a user. Typically, when connected to a DSL switch, the position of a subscriber may usually be defined only in terms of a fiber optic cable connected to the DSL switch. The fiber optic cable may serve a relatively wide geographic area via different multiplexers and manual switching matrixes. Usually WLAN cells are directly connected to private DSL modems. Therefore, even in WLANs the positioning of a user placing an emergency call via the IMS may boil down to the problem of positioning DSL line modems. An average WLAN cell may serve a surprisingly large geographic area and the determination of the position of a user in emergency may be very difficult for the emergency personnel. Similarly, a user connected to a fixed LAN may be difficult to position because the LAN may comprise large buildings and interconnected campuses that are connected via routers and leased lines. National regulators may have different requirements for network operators as to the accuracy of the user position information. In some cases user defined position information may suffice or it may be the only technically feasible option for positioning a user.
Due to the aforementioned factors it would be beneficial to be able to obtain a user defined position for the user from the IMS. It should also be beneficial for the IMS to rely on existing mechanisms for the obtaining of the user defined position.